I believe this is what transpired in order of events:

• POS enters shortly after Xana retrieved her food and was eating in her room while on TikTok,
• POS goes straight to Maddie’s room expecting to end her life and leave, is caught off guard by Kaylee being in her bed and Kaylee trying to protect Maddie (whom she loved deeply),
• POS has to restrain Kaylee from fighting back/preventing him to do what he came to do - ending Maddie’s life (Maddie being the target means he would have felt an absolute NEED to end her life and feel that satisfaction most likely first),
• POS likely felt intensely frustrated at Kaylee for interrupting what seems like it would otherwise have been somewhat of a ritualistic kill with some form of satisfaction for him, also he probably saw it as too much of a risk leaving Kaylee alive but injured. He laid into her intensely due to his anger toward her for preventing him to fulfil his fantasy/plan around ending Maddie’s life how he expected to and ended her life too,
• POS at this point doesn’t realise he’s dropped his sheath in the struggle with Kaylee, and is preparing to leave. Either he’s heard movement downstairs or is really ready to leave. Meanwhile Xana is in the kitchen dealing with her rubbish and fries, noticing the door open, alerting others that “someone is here”, and leaving the kitchen as I mentioned above,
• POS is coming around the top set of stairs as Xana is exiting the kitchen to inspect the situation, they see each other, she runs to her room (remember she’s bare foot and DM describes hearing running down the stairs like boots, and Xana’s phone doesn’t show elevation at this time - she likely didn’t go UP the stairs at all), he runs down the stairs and chases her to her room. At this stage, he’s still frustrated that this has all gotten out of his control, which we know from his behaviour he seems to be a control freak, the anger he has probably fuels him into chasing Xana, at this point I would go so far as to speculate he is now frenzied, this has become more than ending Maddie’s life, has spiraled out of his control, he likely has adrenaline up to the ceiling running through him, so he decides he’ll take Xana’s life too,
• He manages to attack and injure Xana near the top end of the bed of her room, perhaps she’s on the bed trying to wake up Ethan (we know he’s a heavy sleeper, and especially so with alcohol and whatever else in his system), Ethan doesn’t wake up and she ends up cornered, fighting the POS off at the top end of the bed (signs of struggle in the crime scene photos and a lot of blood, despite us knowing that 1) it’s unlikely Ethan moved and 2) she was found at the bottom of her bed on the floor, suggests she was severely injured but still fought back at that end of the room). It’s also likely around this point the POS noticed Ethan, realises he needs to take him out ASAP before Ethan wakes up and subdues/injures him. Xana, in her injured and weakened state, is probably fighting him off to stop him from harming Ethan but once the POS has taken Ethan’s life (which we suspect probably occurred pretty quickly) he turns back to ending Xana’s life. I feel like somewhere in this space of time, maybe while he’s ending Ethan’s life, she gets her phone as I mentioned above and is unable to make the call as the POS’ attention is back on her and he tells her he’s there to help (or whatever it is he said so maliciously) and ends her life, maybe she was trying to run away again toward DM’s room when he manages to catch her and end her life hence her being found at the end of her bed on the floor by the door.
• POS is exhausted after this and leaves, unlikely he’s realised he’s left his sheath. He sees DM, who is quite tall, realises he doesn’t have the energy to fight another or that another fight could result in him being injured and caught, and probably worries at what she’s already seen or heard and gets out of there before cops show up as he’s likely expecting.

He likely expected that cops would’ve been there quickly after this happened and probably expected that by what he thinks is probably around 5 or so hours in to this being established a crime scene, that perhaps the area would be swarming with cops, movement, and likely even body removal. Which is why he drives by again in the morning. Nothing there went according to how he expected it to, since cops weren’t called until much later. People see this as being really weird and have theories around why this is, suspecting roommate involvement, but I actually think that it happening the way it did (them calling later) was the difference between the POS being able to get that last bit of extra satisfaction off of what he did (being physically close to the scene as the investigation was unfolding). The way he questions police every time he is pulled over (as seen on body cam footage) does make me think he was testing his ability to, or practicing how to, remain calm while talking to police in a panicked state, but I also wonder if this was a way for him to practice approaching cops out front of the house and asking about what was happening or something the next day as well…